Sensor interface circuits

ABSTRACT

An interface circuit for a bridge sensor has a switch that connects to a resistive bridge circuit. The resistive bridge circuit includes a first input terminal, a second input terminal, and a pair of resistive branches that connect between the first and second input terminals. Both of the resistive branches include an output terminal. The switch is connected to the first input terminal and is in series with both resistive branches for connecting and disconnecting a voltage source from the resistive branch output terminals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to measurement systems, and moreparticularly to measurement systems employing bridge circuits.

2. Description of Related Art

A bridge circuit generally includes resistors arranged in two parallelbranches between voltage terminals. Each branch generally includes tworesistors arranged in series, an intermediate terminal disposed betweenthe resistors, and a ‘bridging’ branch connecting the intermediateterminals. Typically, one resistor in each parallel branch has a knownresistance, a third resistor in one parallel branch has an adjustablevariable resistance, and a fourth resistor in the other parallel branchhas unknown resistance. By adjusting the variable resistance of thethird resistor such that no current flows through the ‘bridging’ branchthe unknown resistance of the fourth resistor can be determined. This isbecause the ratio of the fixed resistance of the first resistor and thevariable resistance of the third resistor is the same as a ratio of thefixed resistance of the second resistor and the unknown resistance ofthe fourth when no current flows through the ‘bridging’ branch. Byarranging the fourth resistor such that its resistance varies with aparameter of interest, the parameter can be measured by determining theresistance of the fourth resistor.

One challenge to sensors incorporating such bridge circuits is theinfluence of unrelated factors on the measured resistance. For example,some bridge circuits can be susceptible to high impedance leakage due tothe common mode voltage created at the intermediate terminals. Since thecircuit output is not typically a low impedance source, high impedanceleakage can cause output changes that can be confused with resistancechange due to change in the parameter of interest.

Such conventional systems and methods have generally been consideredsatisfactory for their intended purpose. However, there is still a needin the art for improved sensor interface circuits. The presentdisclosure provides a solution for this need.

SUMMARY OF THE INVENTION

A sensor circuit has a switch that connects to a resistive bridgecircuit. The resistive bridge circuit includes a first input terminal, asecond input terminal, and a pair of resistive branches that connectbetween the first and second input terminals. Both the resistivebranches include an output terminal. The switch is connected to thefirst input terminal and is connected electrically in series with theresistive branches for connecting and disconnecting a voltage sourcefrom the resistive branch output terminals.

In certain embodiments, an input lead can be connected between theswitch and the first input terminal. A second input lead can beconnected between a second switch and the second input terminal. Asecond voltage source can be connected to the second input terminalthrough the second switch. It is contemplated that the first and secondswitches can include mechanical switches, relays, or solid-state deviceslike transistors.

In accordance with certain embodiments, the resistive branches caninclude two resistors electrically connected in series and with therespective output terminal disposed between the resistors. A firstoutput lead can be connected to a first of the output terminals. Asecond output lead can be connected to a second of the output terminals.The first and second output leads can have measurement terminalsdisposed on ends opposite the output terminals. A voltage differentialmeasurement module can be connected to the output lead measurementterminals. The voltage differential measurement module can be configuredand adapted for acquiring differential voltage measurements at themeasurement terminals when either or both of the switches are closed,i.e. when either or both of the voltage sources are connected to thebridge circuit.

It is contemplated that in certain embodiments a cable can include atleast one of the output leads and input leads. The cable can include acurrent leakage path at a location along a length of the lead. Ameasurement system can include the first switch, the second switch, andthe voltage differential measurement module. The measurement system canbe configured and adapted for connecting either or both of the voltagesources to the sensor circuit through the cable, determiningdifferential voltage between the measurement terminals of output leads,and calculating magnitude and/or location of the current leakage pathwithin at least one of the input leads and output leads using thedetermined differential voltage.

A method of detecting current leakage within a sensor interface circuitincludes connecting a first voltage source to a first input terminal ofa bridge sensor, determining voltage at a first measurement terminal ofa first output lead connected to a first resistive branch of the bridgesensor, determining voltage at a second measurement terminal of a secondoutput lead connected to a second resistive branch of the bridge sensor,and calculating a first differential between the determined voltages.

In certain embodiments, method can include quantifying leakage currentflow from one of the first and second output leads using the calculatedvoltage differential. The method can include determining a location ofthe leakage current flow from one of the first and second output leadsusing the first differential voltage. Connecting the first voltagesource to the first input terminal can include closing a first switchconnected between the first voltage source and the first input terminal.

In accordance with certain embodiments, the method can includeconnecting a second voltage source to a second input terminal of thebridge sensor, determining again voltages at the first and secondmeasurement terminals, calculating a second differential between thedetermined voltages, and quantifying leakage current from an input leadconnecting the second voltage source to the bridge sensor second inputterminal. Connecting the second voltage source to the second inputterminal can include closing a second switch connected between thesecond voltage source and the second input terminal of the bridgesensor. It is also contemplated that the method can includedisconnecting the first voltage source and quantifying leakage currentfrom an input lead connected between the first switch and the firstinput terminal.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the systems andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a circuit diagram of a sensor interface circuit constructed inaccordance with the present disclosure, showing a bridge sensor, acable, and a measurement system;

FIG. 2 is a circuit diagram of the sensor interface circuit of FIG. 1,showing the circuit in a first configuration wherein a first voltagesource is connected to the bridge sensor; FIG. 3 is a circuit diagram ofthe sensor interface circuit of FIG. 1, showing the circuit in a secondconfiguration wherein a second voltage source is connected to the bridgesensor; and

FIG. 4 is a method of detecting current leakage attributable to currentleakage from leads connected to the bridge sensor of FIG. 1, showingoperations for quantifying and locating current leakages from input andoutput leads within the cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an exemplary embodiment of a sensorinterface circuit in accordance with the disclosure is shown in FIG. 1,and is designated generally by reference character 100. Otherembodiments of sensor interface circuits in accordance with thedisclosure, or aspects thereof, are provided in FIGS. 2-4, as will bedescribed. The systems and methods described herein can be usedaerospace and automotive applications detecting current leakage insensor interface circuits for measuring load, strain, or temperature byway of non-limiting example.

With reference to FIG. 1, sensor interface circuit 100 includes a bridgesensor 110 (shown in dashed outline), a cable 140 (shown in dashedoutline), and a measurement system 160 (shown in dashed outline). Cable140 is connected between bridge sensor 110 and measurement system 160.It is to be understood that bridge sensor 110, cable 140, andmeasurement system 160 can be separate assemblies connected together forpurposes of acquiring measurements of a parameter of interest. Inembodiments, bridge sensor 110 includes precision resistors configuredand adapted for measuring the parameter of interest in an aircraftsystem, cable 140 is a cable assembly such as a wiring harness, andmeasurement system 160 is control circuitry for aircraft electronicsconfigured and adapted acquire measurements of the parameter of interestusing bridge sensor 110.

Bridge sensor 110 is a resistive bridge sensor having a Wheatstonebridge arrangement. Bridge sensor 110 includes a first resistive branch120, a second resistive branch 130, a first input terminal 112, and asecond input terminal 114. First resistive branch 120 and secondresistive branch 130 are connected between first input terminal 112 andsecond input terminal 114 such that first resistive branch 120 andsecond resistive branch 130 are electrically in parallel with oneanother.

First resistive branch 120 includes a first resistor 122, a first outputterminal 124, and a second resistor 126. First resistor 122 is connectedbetween first input terminal 112 and first output terminal 124. Secondresistor 126 is connected between first output terminal 124 and secondinput terminal 114 such that first resistor 122 and second resistor 126are electrically connected in series between first input terminal 112and second input terminal 114.

Second resistive branch 130 is similar to first resistive branch 120insofar as second resistive branch 130 includes a third resistor 132, asecond output terminal 134, and a fourth resistor 136. Third resistor132 is connected between first input terminal 112 and second outputterminal 134. Fourth resistor 136 is connected second output terminal134 and second input terminal 114 such that third resistor 132 andfourth resistor 136 are connected electrically in series with oneanother between first input terminal 112 and second input terminal 114.At least one of the resistors of bridge sensor 110, e.g. first resistor122, second resistor 126, third resistor 132, and fourth resistor 136,is a variable resistor having resistance that is changeable in responseto change in a parameter of interest, such load, strain, or temperatureby way of non-limiting example.

In embodiments, at least one of first resistor 122, second resistor 126,third resistor 132, and fourth resistor 136 has variable resistance thatvaries with a parameter of interest. The remaining resistors have fixedresistance. In certain embodiments, e.g. in pressure sensor circuits,each of first resistor 122, second resistor 126, third resistor 132, andfourth resistor 136 has variable resistance that varies with a parameterof interest.

Cable 140 includes four leads coupled between bridge sensor 110 andmeasurement system 160. Cable 140 includes a first input lead 142, asecond input lead 148, a first output lead 144, and a second output lead146 each with opposed sensor and measurement system ends. First inputlead 142 is connected to first input terminal 112 on its sensor end.Second input lead 148 is connected to second input terminal 114 on itssensor end. First output lead 144 is connected to first output terminal124 on its sensor end. Second output lead 146 is connected to secondoutput terminal 134 on its sensor end.

Measurement system 160 includes a first voltage source 162, a voltagedifferential measurement module 164, and a second voltage source 166.First voltage source 162 is connected to first input terminal 112through first input lead 142. Second voltage source 166 is connected tosecond input terminal 114 through second input lead 148. Differentialvoltage measurement module 164 is connected to first output terminal 124and second output terminal 134 through first output lead 144 and secondoutput lead 146, respectively. In embodiments, differential voltagemeasurement module 164 includes a differential amplifier or comparatorcircuitry configured adapted to output a voltage differential betweenvoltages observed between measurement ends of first output lead 144 andsecond output lead 146, i.e. at ends opposite first output terminal 124and second output terminal 134. Differential voltage measurements can beobtained by (a) acquiring a first single-ended voltage measurementbetween the end of first output lead 144 and ground 10, (b) acquiring asecond single-ended voltage measurement between the end of second outputlead 146 and ground 10, and (c) determining a voltage differential bysubtracting the first single-ended measurement from the secondsingle-ended measurement. It is contemplated that the determination ismade using software resident on a memory communicative with a processoroperatively associated with measurement system 160.

Cable 140 includes electrical insulation 149. Some cable insulationmaterials can break down in the presence of substances like water,hydraulic fluid, deicing fluid, or fuel, thereby developing a currentleakage path from conductors disposed within the cable. Such currentleakage paths can result in stray or spurious signals being emitted bythe cable into aircraft structure, potentially reducing reliability. Forpurposes of illustration, FIG. 1 shows leakages paths from each leadwithin lead of cable 140, a first input lead current leakage path R₁₄₂extending from first input lead 142 to a ground terminal (illustrated indashed lines), a second input lead current leakage path R₁₄₈ extendingfrom second input lead 148 to a ground terminal (illustrated in dashedlines), a first output lead current leakage path R₁₄₄ extending fromfirst output lead 144 to a ground terminal (illustrated in dashedlines), and a second output lead current leakage path R₁₄₆ extendingfrom second output lead 146 to a ground potential terminal (illustratedin dashed lines). It is to be appreciated and understood that any orcombination of leads within cable 140 can include a current leakage pathbetween the lead and a ground terminal. It is also to be understood thatthe resistance of such leakage path is on order of many kilo Ohms toseveral mega Ohms.

Measurement system 160 also includes a first switch 168 and a secondswitch 170. First switch 168 is connected between a first voltage source162 and first input lead 142 and is configured and adapted forelectrically connecting and disconnecting first voltage source 162 fromfirst input terminal 112. Second switch 170 is connected between asecond voltage source 166 and second input lead 148 and is configuredand adapted for electrically connecting and disconnecting second voltagesource 166 from second input terminal 114. First switch 168 and secondswitch 170 can be mechanical switches or relays. In certain embodiments,first switch 168 and second switch 170 are solid-state devices such asMOSFETs or IGBTs that are operatively associated with a controllercommunicative with a memory having instructions recorded thereon toelectrically open and close the switches in accordance with thebelow-described method.

With reference to FIG. 2, sensor interface circuit 100 is shown in afirst configuration. In the first configuration, first switch 168 iselectrically closed and second switch 170 is electrically open position.This causes current to flow through first input lead 142, first resistor122, third resistor 132, first output lead 144, and second output lead146. When first output lead current leakage path R₁₄₄ and second outputlead current leakage path R₁₄₆ are zero or equivalent to one another,voltage V_(A(1)) and V_(B(1)) will be equivalent, and a differentialvoltage measurement V_(Diff(1)) reported by differential voltagemeasurement module 164 will be zero.

In the event that first output lead current leakage path R₁₄₄ and secondoutput lead current leakage path R₁₄₆ are unequal to one another,voltage V_(A(1)) and V_(B(1)) will be unequal. This will causedifferential voltage measurement V_(Diff(1)) reported by differentialvoltage measurement module 164 to be non-zero. The magnitude of theleakage current will be indicated by the magnitude of differentialvoltage measurement V_(Diff(1)). Location of the current leakage, e.g.whether current leakage from one of first output lead 144 and secondoutput lead 146 is greater than the other of the first output lead 144and second output lead 146, will be indicated by the polarity ofdifferential voltage measurement V_(Diff(1)).

With reference to FIG. 3, sensor interface circuit 100 is shown in asecond configuration. In the second configuration, first switch 168 iselectrically open and second switch 170 is electrically closed. Thiscauses current to flow through second input lead 148, third resistor132, fourth resistor 136, first output lead 144, and second output lead146. As with the first configuration, when first output lead currentleakage path R₁₄₄ and second output lead current leakage path R₁₄₆ arezero or equivalent to one another, voltage V_(A(1)) and V_(B(1)) will beequivalent, and a differential voltage measurement V_(Diff(1)) reportedby differential voltage measurement module 164 will be zero. Similarly,if that first output lead current leakage path R₁₄₄ and second outputlead current leakage path R₁₄₆ are unequal to one another, voltageV_(A(1)) and V_(B(1)) should be unequal. Specifically, differentialvoltage measurement V_(Diff(2)) reported by differential voltagemeasurement module 164 should be about the same as differential voltagemeasurement V_(Diff(1)) to be non-zero.

In the event that differential voltage measurement V_(Diff(2)) is notequivalent to voltage differential measurement V_(Diff(1)), then atleast one of first input lead current leakage path R₁₄₂ and second inputlead current leakage path R₁₄₈ is non-zero. For example, if there iscurrent leakage through second input lead current leakage path R₁₄₈while first switch 168 is closed and second switch 170 is open, secondinput lead current leakage path R₁₄₈ forms a leg of a voltage dividercircuit. This causes differential voltage measurement V_(Diff(1)) to beunequal to V_(Diff(2)). Similarly, if there is current leakage throughfirst input lead current leakage path R₁₄₂ while second switch 170 isclosed and first switch 168 is open, first input lead current leakagepath R₁₄₂ forms a leg of a voltage divider circuit. This likewise causesdifferential voltage measurement V_(Diff(2)) to be unequal toV_(Diff(1)). Under these circumstances, location and magnitude currentleakage from the input leads can be determined by comparing an output ofdifferential voltage measurement module 164 when both first switch 168and second switch 170 to either differential voltage measurementV_(Diff(1)) and voltage differential measurement V_(Diff(2)).

With reference to FIG. 4, a method 200 of detecting current leakagewithin a sensor interface circuit, e.g. bridge sensor 110 (shown in FIG.1), is shown. Method 200 includes a step 210 for connecting a firstvoltage source, e.g. first voltage source 162 (shown in FIG. 1) to afirst input terminal of the bridge sensor, e.g. first input terminal 112(shown in FIG. 1). Connecting the first voltage source to the firstinput terminal can include closing the first switch connected betweenthe first voltage source and the first input terminal.

Method 200 also includes a step 220 for determining voltage at a firstmeasurement terminal of a first output lead connected to a firstresistive branch of the bridge sensor, e.g. first output lead 144 (shownin FIG. 1), and a step 230 for determining voltage at a secondmeasurement terminal of a second output lead connected to a secondresistive branch of the bridge sensor, e.g. second output lead 146(shown in FIG. 1). Method 200 additionally includes a step 232 forcalculating a first differential between the determined voltages, a step234 for quantifying leakage current flow from one of the first andsecond output leads using the calculated voltage differential, and astep 236 for determining a location of the leakage current flow from oneof the first and second output leads using the first differentialvoltage.

In embodiments, method 200 includes a step 240 for connecting a secondvoltage source, e.g. second voltage source 166 (shown in FIG. 1) to asecond input terminal, e.g. second input terminal 114 (shown in FIG. 1)of the bridge sensor, a step 242 for determining again voltages at thefirst and second measurement terminals, a step 244 for calculating asecond differential between the determined voltages, and a step 246 forquantifying leakage current from an input lead, e.g. second input lead148 (shown in FIG. 1), for connecting the second voltage source to thebridge sensor second input terminal. Connecting the second voltagesource to the second input terminal can include closing a second switch,e.g. second switch 170 (shown in FIG. 1), connected between the secondvoltage source and the second input terminal of the bridge sensor. It isalso contemplated that the method can include a step 248 fordisconnecting the first voltage source and a step 250 for quantifyingleakage current from an input lead connected between the first switchand the first input terminal. As indicated by reference numeral 270,method 200 can be iteratively repeated to monitor measurement accuracyover time.

Conventional bridge-type sensors can be susceptible to high impedanceleakage due to the common mode voltage created at the sensor outputs. Asthe sensor output is not typically a low impedance source, this cancause error in measurements acquired using the sensor as the sensoroutput can be degraded by the unintended leakage path. Embodiments ofsensor interface circuits described herein allow evaluating differentialleakage between the sensor outputs. Determining differential leakagebetween the sensor outputs allows for determining relative magnitude ofthe leakages. Once determined, sensor output can be adjusted in view ofthe relative magnitudes of the leakages, thereby compensating forpotentially degraded measurement system performance and avoidingmeasurement inaccuracy.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for bridge sensor interface circuitswith superior properties including increased accuracy. While theapparatus and methods of the subject disclosure have been shown anddescribed with reference to preferred embodiments, those skilled in theart will readily appreciate that changes and/or modifications may bemade thereto without departing from the spirit and scope of the subjectdisclosure.

What is claimed is:
 1. A sensor interface circuit, comprising: first andsecond input terminals; a resistive bridge circuit including a pair ofparallel resistive branches connected between the first and second inputterminals, both resistive branches including an output terminal disposedbetween serially arranged resistors of the resistive branches; and aswitch connected to the first input terminal and in series with theresistive branches for connecting a voltage source to both resistivebranch output terminals.
 2. A circuit as recited in claim 1, furtherincluding a voltage source connected to the first input terminal throughthe switch.
 3. A circuit as recited in claim 2, wherein the voltagesource is a first voltage source and further including a second voltagesource connected to the second input terminal.
 4. A circuit as recitedin claim 1, wherein the switch is a first switch and further including asecond switch connected to the second input terminal.
 5. A circuit asrecited in claim 4, wherein the second switch is connected in serieswith both of the parallel resistive branches.
 6. A circuit as recited inclaim 1, further including a voltage differential measurement moduleconnected to the resistive branch output terminals.
 7. A circuit asrecited in claim 6, further including an output lead connected betweenan output of the first resistive branch and the voltage differentialmeasurement module.
 8. A circuit as recited in claim 7, furtherincluding an input lead connecting the switch to the voltagedifferential measurement module through the output lead.
 9. A method ofdetecting current leakage within a sensor interface circuit, comprising:connecting a first voltage source to a first input terminal of a bridgesensor; determining voltage at a first measurement terminal of a firstoutput lead connected to a first resistive branch of the bridge sensor;determining voltage at a second measurement terminal of a second outputlead connected to a second resistive branch of the bridge sensor; andcalculating a first differential between the determined voltages.
 10. Amethod as recited in claim 9, further including quantifying leakagecurrent flow from one of the first and second output leads using thecalculated voltage differential.
 11. A method as recited in claim 9,further including determining a location of the leakage current flowfrom one of the first and second output leads using the firstdifferential.
 12. A method as recited in claim 9, wherein connecting thefirst voltage source to the first input terminal includes closing afirst switch connected between the first voltage source and the firstinput terminal of the bridge sensor.
 13. A method as recited in claim 9,further including: connecting a second voltage source to a second inputterminal of the bridge sensor; determining voltage at the firstmeasurement terminal; determining voltage at the second measurementterminal; calculating a second differential between the determinedvoltages; quantifying leakage current from an input lead connecting thesecond voltage source to the bridge sensor second input terminal.
 14. Amethod as recited in claim 13, wherein connecting the second voltagesource to the second input terminal includes closing a second switchconnected between the second voltage source and the second inputterminal of the bridge sensor.
 15. A method as recited in claim 13,further including: disconnecting the first voltage source; andquantifying leakage current from an input lead connected between thefirst switch and the first input terminal.